Identification systems exist in which persons, objects, animals or the like each equipped with a transponder, can be identified by means of interrogation devices. In each transponder a message is stored which uniquely identifies the person, object or animal and which in response to reception of a high frequency interrogation pulse is transmitted by the transponder. This renders the interrogation device able to receive and evaluate the message.
A concrete example for such an identification system is a system in which animals, for example pigs housed in a pigsty, are each provided with a transponder in the form of an implanted earmark. The intention of the identification system is to make it possible, even with a large number of animals, to exactly identify each individual animal at any time to permit exact supervision of the individual animals. In this example of use in the pigsty several fixedly installed interrogation devices are in operation which in recurring intervals transmit HF interrogation pulses which can be received by the transponders which happen to be within range of the transmitter of the interrogation device. Since the range is only limited it is only in exceptional cases that several animals are simultaneously present within the range. It can for example be ensured by constructional steps within the pigsty that in fact always only one animal is within the transmitter range.
However, in addition to the fixedly installed interrogation devices, hand devices may also be used with which the transponders of individual animals can be interrogated quite specifically by bringing the hand device into the vicinity of the animal of interest and then transmitting the HF interrogation pulse.
In the example outlined the transponders have very small dimensions and are batteryless devices which derive their supply power solely from the received HF interrogation pulse. This means, however, that the transmission power for transmitting the message stored in them is very small so that accordingly only a small range can be achieved. Since the transmission power of the stationarily arranged interrogation devices and of the hand devices can be substantially greater because of the presence of their own power supply means, by suitable steps it must be ensured that the messages transmitted by the transponders cannot be concealed by the HF interrogation pulses, thus then not being received by the interrogation devices. Such a step would be to synchronize the interrogation devices with each other in such a manner that they transmit the HF interrogation pulses in each case exactly simultaneously so that the transponders can send their message to the interrogation device lying closest to the transponder in the pauses between said pulses. However, this solution requires that all the interrogation devices operate in a fixed time pattern in which the time interval between the HF interrogation pulses must be set so that the most unfavourable transmission case is still also taken into account. However, precisely in the example outlined above, it can occur that animals moving rapidly past the interrogation devices are not detected because the pauses between the interrogation pulses are too long.
The problem underlying the invention is to provide a method of the type outlined at the beginning which with any desired number of interrogation devices and transponders in an interrogation area permits rapid and reliable message transmission from the individual transponders to the interrogation devices without the latter mutually influencing each other.
According to the invention, this problem is solved in that each interrogation device as a reaction to an interrogation command checks whether its receiving section is receiving a signal with the carrier frequency. Then, if no such signal is being received, interrogation device transmits the HF interrogation pulse after expiry of a wait time of at least the predetermined message duration. However, when such a signal is being received the interrogation device does not transmit the HF interrogation pulse until the reception of said signal terminates and at least the wait time has expired or a delay time has expired which is longer than the sum of the predetermined carrier duration and the wait time.